1. Field of the Invention
This invention relates to the manufacture of semiconductor devices, and more specifically to the process of assembling an integrated circuit into a package.
2. Related Art
Laser fuses have been used in the electronics industry to repair memory elements, configure logic circuits, and customize integrated circuits such as gate arrays by selectively removing desired fuses in the device. For example, a non-functional device can be repaired by removing desired fuses, e.g., ablation by laser, to isolate defective portions of the circuitry or to substitute functionally redundant circuitry for the defective portions. Fuses can also be used to mark the device for identification of characteristics in a manner that is readable visually or electrically, e.g., serialization of the integrated circuit or how the device has been configured by the laser. An integrated circuit can be customized or configured for specific uses by altering the structure, path or electrical characteristics of the device or elements through selective removal of the fuses. It should be noted that the word "fuse" can refer to an antifuse as well as a fuse.
The fuses can be removed at various stages of the integrated circuit manufacturing process. By removing the fuses after the wafer has been sawed up into individual integrated circuits and assembled into packages, the lead time to deliver customized or configured integrated circuits, hereinafter referred to as die, can be reduced. A typical process for assembling a die into a package and configuring the die in the package includes the following basic steps:
1. deposit die-attach epoxy on a lead frame in the cavity of the package, where the cavity can be formed at the time the package is created or at a later time; PA1 2. place the die on the epoxy and compress the die and the package together to distribute the epoxy; PA1 3. bake the package and die to cure the epoxy; PA1 4. attach bond wires between bonding pads on the die and lead fingers on the package; PA1 5. configure the die with the use of a laser; and PA1 6. seal off the top of the package, either with a lid or by filling in the cavity with mold compound.
A major factor limiting the successful application of the laser and configuration of the die in this manner is the difficulty with aligning and focusing the laser if the die does not sit evenly on the lead frame. A die 10 typically has alignment and focusing marks or targets 1-4 on the die's upper surface, as shown in FIG. 1. A typical alignment and focusing sequence for die 10 would begin with a laser scanning targets 1 though 4 one at a time and making x-axis, y-axis, and rotational corrections. The laser then returns to target 1 and makes another series of scans through to target 4 to set the focus level for the entire die 10.
Alignment and focusing in this manner is adequate as long as the die is set evenly upon the lead frame so that the laser scanning plane is parallel with the surface of the die. In other words, as long as any non-orthogonality in the plane of the die, relative to the optics of the laser, does not cause the lasered or scanned portion of the die to fall outside of the laser spot's focus range, proper alignment and focusing can be achieved. However, when the die is not properly placed on the lead frame, the laser may be unable to perform an alignment or focus scan or to accurately ablate the fuses for proper circuit customization.
FIG. 2 shows a die 10, which has been placed and compressed on some die-attach epoxy 20. Epoxy 20 has been deposited on a lead frame (not shown) within a cavity of a package 21. Several factors can cause die 10 to sit at an angle relative to package 21, including the presence of air bubbles 22 within epoxy 20, an uneven distribution of epoxy 20, or an uneven placement of die 10 on epoxy 20. This will often result in a portion of die 10 falling out of the focus range of the laser, which can lead to improper configuration of the die.
Although various leveling mechanisms are common in the industry and would be effective in re-leveling the die, these types of mechanisms are not commonly present in commercially-available lasers used for the repair and configuration of semiconductor devices. Therefore, using these mechanisms to re-level the die so that reliable configuration of the die in a package is possible would require costly and time-consuming modifications to the laser. Accordingly, it is desirable to have a method of accurately adhering a die to a package so that re-leveling the die is unnecessary, which eliminates the additional expenses required to modify the laser for re-leveling.